Loudspeaker



Filed Aug. 29, 1956 INVENTOR ERICH TH IEN HAUS ATTORNEY Patented Apr.26, 1938 UNITED STATES LOUD SPEAKER Erich Thienhaus, Berlin, Germany,assignor to Telefunken Gesellschaft fiir Drahtlose Telegraphic in. b.IL, Berlin, Germany, a corporation of Germany Application August 29,1936, Serial No. 98,457 In Germany June 8, 1935 6 Claims.

The present invention is concerned with a special design of casing foracoustic apparatus, especially for loudspeakers and microphones.

H In the operation of a piston-type diaphragm which vibrates freely inspace and which represents a projector or radiator of the first order,an acoustic short-circuit is produced upon both sides of the diaphragmfor low frequencies for reasons of pressure equalization. The result of10 this action is that these particular frequencies are not radiated toany appreciable extent and do not therefore contribute to the faithfulreproduction of the sound to be translated. It is known from the priorart that the said acoustic short-circuit is avoidable either by havingthe diaphragm built into an infinitely large acoustic baffle or else byhaving the posterior face of the diaphragm completely shut or enclosedso that equalization of pressure is precluded between the pressure wavesradiated or projected by the anterior face of the diaphragm and thepressure waves so projected from the posterior face thereof. What thusresults is a radiator or projector of order. A projector of the 0 orderis a sound source at which the energy is at all sides radiated from theinterior to the exterior. Its fundamental shape is represented by asphere the surface of which borders the medium in question (air, water),said surface performing in all its points co-phasal and radialoscillation movements of equal amplitude due to the active, periodicalforces.

However, inasmuch as the size of the acoustic bafile, in most instances,is limited or finite, and since, moreover, a closed box or case wouldtend to greatly raise the restoring force and thus the natural frequencyof the diaphragm, a fact that would make itself felt in a verydisagreeable manner in high-fidelity loud-speakers which inherentlypossess a very low natural period, the above mentioned two methods ofremedying the situation may be used in practice only conditionally. Thecompromise method which has been suggested for relatively small-sizedloudspeaker models, namely, to fit the loudspeaker in an open casing,however, fails to reduce completely the undesirable pressureequalization between the front and the back of the diaphragm. Inaddition, it has the drawback that reproduction is seriously impairedbecause of dissimilarities in box resonances.

In order to avoid the latter, it has been also suggested to mount withinthe casing, when the loudspeaker is built into a wall, another diaphragmwhich is capable of freely vibrating in this wall. This second diaphragmis only set in motion by the volume of air confined inside the casingand which is set in oscillatory motion by the driven diaphragm. Withthis form of construction, it is almost possible to suppress theacoustic short-circuit, but this form of construction involves thedrawback that the very large dimensions of the casing must be chosenwhen the diaphragms are disposed in superposition. But if the seconddiaphragm is fitted in the rear of the casing, then the said seconddiaphragm must be afforded a chance to radiate freely. In other words,the casing should not be mounted directly on a wall, and the result isthat the space requirement becomes rather considerable also for thistype of loudspeaker.

Now, in order that such drawbacks as exist in the designs known in theearlier art may be avoided, pressure equalization or compensation, bycomplete closureof the posterior face of the diaphragm, is insuredaccording to this invention, without an increase in restoring force.This is brought about not by having the enclosure or chamber situatedposteriorly of the diaphragm filled with air (which, at normaltemperatures is in normal gaseous state so that, for acoustic actions,the law of adiabatic change of state is obeyed), but by having theenclosure filled with a saturated vapor so that with each increase orreduction in the enclosure volume there results an evaporation orcondensation in the absence of pressure changes. No matter what thecircumstances, there prevail within the chamber the saturation pressuresof the vapor in question at the prevailing temperature. For instance,the chamber could be filled with vapor to the 100- percent limit andcould operate exactly at boiling point, or else a small residual volumeof air or the like neutral gas is provided and one works somewhat belowthe boiling temperature. In this latter instance, the state ofsaturation which corresponds to the temperature which happens to prevailwill be assumed automatically or spontaneously. The pressure inside thechamber is advantageously chosen always equal to the barometric pressureof the air. This automatic pressure equalization may be insured, forexample, by the aid of a U-shaped pipe or other device filled with wateror some other liquid. The pressure equalization is accomplishable also,for instance, by the aid of small bellows which follows or responds tothe pressure of the outside atmosphere.

What should primarily be considered for the purpose of filling thechamber are gases or vapors of liquids which will liquefy in theneighborhood of normal room temperature. Particularly suited for theobject have been found vapors of organic gases such as acetaldehyde,pentane, ethyl bromide, and the like, the boiling points of which, atatmospheric pressure, are in the neighborhood of to 40 degrees C. With aview to insuring more reliably a certain temperature interval or range,it may occasionally be advisable to resort to the heating of thechamber, for instance, by

the aid of a glow-lamp, a glow-tube or gaseousdischarge tube, a heaterresistance, etc. Where the magnet systems of loudspeakers are energizedfrom an outside or separate source of energy, it may turn out to beadvantageous to use the rectifier or the magnet system to act as aheater element.

For low temperatures, evaporation and condensation occur in each periodin accordance with this formula:

m: mmax.SlI1 wt where m is the amount or volume of medium in vaporousstate existing at each instant contrasted with the normal or quiescentstate; in other words, if merely the state of change occurring insidethe chamber is taken into consideration, the evaporation amplitude or,with opposite sign, the condensation amplitude. Inasmuch as thetemperature, in the evaporation and condensation process, stays stableor constant, it follows that also the pressure prevailing inside thechamber stays constant during the motion of the diaphragm, that is,equal to the saturation pressure of the vapor.

Hence, the process or action which takes place during the motion of thediaphragm inside the chamber is as follows: If the diaphragm is movingin the direction towards the interior of the chamber, in other words, ifthe volume of the chamber is reduced, a volume of gas corresponding tothe displaced volume will be precipitated or condensed in liquid form,in other words, this quantity of gas is condensed. But when thediaphragm moves in the opposite direction, the volume of the chamberincreases. As a result of this growth in volume, a correspondingquantity of liquid gets a chance to evaporate again and to fill thisvolume.

Inasmuch as during the inward motion of the diaphragm, part of the gasbecomes condensed, no compression of the gas happens, for the volume ofthe condensate is negligibly small compared with the Vapor volume. Inthe operation of the acoustic apparatus, this, as will be seen, has theeffect that no increase in restoring force is caused. In other words,what is thus obtained is a sound radiation characteristic of a radiatoror sound projector of the order which, as is well known, is particularlyfavorable for the low frequencies, without the drawback of anundesirable increase in restoring force being occasioned, a conditionwhich is inseparable from closed chambers filled with air.

In the practical construction of such loudspeakers, the chamber locatedposteriorly of the diaphragm may be chosen extremely small. In fact, theclosure need be spaced from the diaphragm only to such an extent thatwhen the diaphragm experiences a maximum. deflection, hitting of theclosure wall by the diaphragm will be. safely avoided. In other words,it is thus feasible to use extremely reduced casing dimensions inacoustic apparatus such as loudspeakers or microphones.

It will be seen that the time available for the occurrence ofevaporation and condensation becomes proportionately less as thefrequency rises, with the result that a slight lag becomes noticeable inboth the condensing and evaporating actions, with the further resultthat eventually a pressure variation of p=pn.sin (wt+) becomes added,where inn is the pressure amplitude and the phase shift. Under practicalconditions this has as its physical efiect that in the presence of thelow frequencies no restoring power is present, for the reason that forlow frequencies adequate time is available for the proper production ofthe condensation and evaporation processes. However, restoring powermakes itself manifest in proportion as the frequency grows as aconsequence of the lag or delay produced in the condensing andevaporating actions, and this restoring force will grow gradually. Thisparticular phenomenon could be used for the purpose of ininfluencing orgoverning the frequency response.

The annexed drawing shows an exemplified embodiment of the invention.

Confined inside a casing I is the drive system 2 and diaphragm 3. Thecasing, in this particular form of construction, embraces the posteriorpart of the loudspeaker diaphragm so that there is but a small distancebetween them. Placed inside the casing or chamber I is the cup 4containing the liquid to be evaporated. Between the interior of thecasing I and the outside air, pressure equalization is insured by Way ofthe valve 5. Case I is mounted upon a foot or base 6.

What I claim is:

1. Acoustic apparatus provided with a diaphragm and means for actuatingsaid diaphragm, an enclosure completely enclosing the back of thediaphragm and forming therewith an air-tight chamber, and a saturatedvapor within said chamber maintained at atmospheric pressure whereby anegligibly low restoring force is imparted to the diaphragm.

2. Acoustic apparatus according to the invention defined in claim Iwherein the saturated vapor is produced from a liquid that is capable ofvaporizing at room temperature.

3. Acoustic apparatus according to the invention defined in claim 1wherein the chamber contains a small volume of a neutral gas in additionto the saturated vapor.

4. Acoustic apparatus provided with a diaphragm and means for actuatingsaid diaphragm, an enclosure completely enclosing the back of thediaphragm and forming therewith an air-tight chamber, and means withinthe chamber for imparting a negligibly low restoring force to thediaphragm, said means including a vapor of an organic gas of the classincluding acetalydehyde, pentane, and ethyl bromide, which vapor ismaintained at atmospheric pressure.

5. Acoustic apparatus provided with a diaphragm and means for actuatingsaid diaphragm, an enclosure completely enclosing the back of thediaphragm and forming therewith an air-tight chamber, a saturated vaporwithin said chamber maintained at atmospheric pressure whereby anegligibly low restoring force is imparted to the diaphragm, and meansincluding a U-shaped tube having a liquid therein communicating with thechamber whereby the pressure therein is automatically equalized withrespect to atmospheric pressure.

6. Acoustic apparatus provided with a diaphragm and means for actuatingsaid diaphragm, an enclosure completely enclosing the back of thediaphragm and forming therewith an air-tight chamber, a saturated vaporwithin said chamber maintained at atmospheric pressure whereby anegligibly low restoring force is imparted to the diaphragm, and heatingmeans within the chamber whereby the vapor is maintained in a state ofsaturation.

ERICI-I THIENHAUS.

